The systematic development of membrane potential sensitive dyes has made it possible to measure a cell's electric potential through measuring its fluorescence response. The conventional measurement of one-photon fluorescence, however, allows only limited (~10%) accuracy. Measurements of the two-photon emission shift show similar accuracy. We are continuing to measure the two-photon excitation spectrum in the hope that the different selection rules for two-photon excitation trigger a large change in the cross-section in response to the application of a symmetry-breaking electric field.